The present invention relates to techniques for delivering power over a data communications cable, such as a category 5 cable or equivalent, and more particularly, to a system and method for characterizing a load at the end of a communication cable prior to delivering power to the load.
In Power over Ethernet (PoE) systems, it is desirable to determine the resistance of a load within a powered device (PD) that is powered by power sourcing equipment (PSE) to ascertain whether the PSE should apply power to the load. This determination can be complicated for a number of reasons. First, a communications cable coupling the PSE to the PD may be coupled directly to the load or alternatively, may be coupled to the load via a diode rectifier or a diode bridge. Moreover, if the load is shunted by a large capacitance, a DC measurement may be distorted since the voltage across the shunt capacitance may not stabilize prior to completion of the measurement. Additionally, as the resistive load is typically within the PD at the end of the communication cable, which may extend a considerable distance, the load measurement is susceptible to noise pickup from the cable.
The need to determine whether a PD includes a load which can be powered is becoming more and more widespread as an increasing number of PDs are deployed using PoE. It would therefore be desirable to have a system and method which allows characterization of a load within a PD in a manner which is reliable and inexpensive so as not to burden the product costs of PSE equipment.